1. Field of the Invention
The present invention relates to a rear projection screen comprising a Fresnel lens and a lenticular lens.
2. Related Art
Rear projection screens that comprise Fresnel lenses for refracting rays projected from an imaging source to make them almost parallel with each other, and lenticular lenses for scattering projected rays to form an image have conventionally been known as screens useful for rear projection type TVs and the like.
Further, among the conventional rear projection screens, there has been known a screen in which projected rays are diffused in the horizontal direction by a lenticular lens sheet for horizontal diffusion, and in the vertical direction by both a diffuser and a lenticular lens sheet for vertical diffusion. It is noted that the diffusion angles of these lenticular lens sheets are generally uniform over the entire screen surface.
In the conventional rear projection screens, the diffusion angle is uniform over the entire screen surface, so that there is such a problem that the phenomenon of shading, which will be described hereinafter, tends to occur.
FIGS. 12A, 12B and 12C are views for illustrating the phenomenon of shading that occurs on a rear projection screen. In these figures, FIG. 12A is a view showing how those rays projected from an imaging source 20 are observed on a rear projection screen 10. In this figure, the intensity and direction of diffusion at three points a, b and c on the rear projection screen 10 are indicated by the length and direction of an arrow, respectively. Further, FIGS. 12B and 12C respectively show the luminance distributions on the rear projection screen 10, obtainable by observing the screen from observation points p1 and p2.
As can be understood from FIGS. 12A and 12B, when the rear projection screen 10 is observed from the front (observation point p1), the luminance at the marginal part (edges a and c) of the rear projection screen 10 is lower than that at the central part b of the rear projection screen 10, so that the marginal part of the rear projection screen 10 is perceived as dark. Further, as can be known from FIGS. 12A and 12C, when the rear projection screen 10 is observed from a position (observation point p2) deviated from the position in front of the screen, the luminance at the edge a which is near the observation point p2 is highest, and this luminance gradually decreases as the distance from the edge a increases toward the central part b and then toward the edge c which is at the opposite of the edge a. Therefore, those parts on the rear projection screen 10 which are apart from the observation point p2 are perceived as dark.
In such a rear projection screen 10, the diffusion angle in the vertical direction is generally set to be narrower than that in the horizontal direction. Therefore, the phenomenon of shading tends to occur in the vertical direction on the rear projection screen 10, and the brightness on the rear projection screen 10 has thus lacked uniformity.
A method in which the focal length of the Fresnel lens constituting the rear projection screen 10 is decreased has been known as a method for reducing the occurrence of such a shading phenomenon. By this method, the point of focus on the observation side of the Fresnel lens (i.e., the observation side of the rear projection screen 10) is made closer to the lens, so that, in FIG. 12A, a ray of light projected on the marginal part (edges a and c) of the rear projection screen 10 emerges by being inclined to the inside. For this reason, in this rear projection screen 10, increased luminance is observed on the marginal part (edges a and c) of the rear projection screen 10 when the screen is observed from the front (observation point p1). Moreover, even when the screen is observed from a position (observation point p2) deviated from the position in front of the screen, improved uniformity of brightness is attained on the rear projection screen 10 because decreased luminance is observed at the edge a which is near the observation point p2, while increased luminance is observed at the edge c which is apart from the observation point p2.
However, in such a rear projection screen 10, when the focal length of the Fresnel lens constituting the rear projection screen 10 is decreased, the difference among the exit angles of imaging rays of three colors projected from different positions on the imaging source 20 becomes large. When the difference in exit angle in terms of vertical direction against which the diffusion angle of the rear projection screen 10 has been set narrow becomes large, color shading tends to occur on the rear projection screen 10. When this color shading is taken into consideration, it is difficult to make the focal length of the rear projection screen 10 on the observation side shorter than approximately 10 m. Therefore, it has been impossible to reduce the occurrence of the shading phenomenon without bringing about color shading.